In patients with inflammatory bowel disease using a maintenance dose of sulfasalazine (SASP), compliance to therapy was studied by measuring serum sulfapyridine (SP) levels. Serum SP levels were determined in 51 patients both during hospitalization and at outpatient follow-up 1-6 months later. In 21 patients (41.2%) the serum SP level was considerably lower at outpatient follow-up than the serum level before discharge. In 21 (12%) of 175 outpatients taking a maintenance dose of SASP, SP in serum was not detectable at all at repeated determination during follow-up. These results indicate that a substantial number of patients on maintenance therapy with SASP do not take the prescribed dose. Drug trials in these patients should therefore incorporate methods of detecting defaulters.